Add canceling of mouse movement to MouseEvent

Like the title says, expand the MouseEvent to allow mod creators to cancel the mouse movement (zeroing the deltas).

The main point to this would be to lock a player's rotation when they try to move their mouse.

• CameraSetupEvent only affects the camera's rotation, as the player can spin freely as they wish. That is a no-go.
  
• "Teleporting" the player back to a rotation looks hideous. It's jittery, and awful, and I won't be a part of that crowd.
  

Alternatives include:

• Release the final modifiers on the delta x and y variables in the event, so you could just set them to 0 when the event is fired.
  
• Create a new PlayerRotateEvent, which fires on both client, and server, allowing flexibility on each end when either the mouse moves, or the server is about to rotate the player via yaw and pitch updates
  

Opening this up would allow mod creators to do more immersive "cut scenes", or animations and renderings that look better when the player is not in turning. For example, when I was working on a hookshot mod, it looked really weird to fire it, and the player could turn around while holding it as the spike was pulling chain away from them. Locking the player rotation would help with effects like that.

I would even be alright with a new EntityPlayer#setRotationLocked(Object mod,boolean locked).

The purpose to having the mod instance would be to allow several mods to lock the rotation for their own purpose, but not unlock it later when another mod needs it locked.